Electrical heating devices



Oct. 31, 1961 G. v. wooDLlNG ELECTRICAL HEATING DEVICES 4 Sheets-Sheet 1 Original Filed Oct. 21, 1948 vill, 111, D

L /46 /A/5 U1. A T/o/v BY QQVENTZR. y@ MEE# Oct. 31, 1961 G. v. wooDLlNG 3,007,026

ELECTRICAL HEATING DEVICES original Filed oct. 21, 1948 l 4 sheets-sheet 2 76 @WPI/r AL TER/VA 7'//VG CURRENT I SOURCE l BM (VINVENTOR. Mmmm w1 Oct. 3l, 1961 G. v. wooDLlNG 3,007,026

ELECTRICAL HEATING DEVICES Original Filed Oct. 21, 1948 4 Sheets-Sheet 3 ALTERNAT/NG AL TER/VAT/ Oct 31, 1961 G. v. WOODLING 3,007,026

ELECTRICAL HEATING DEVICES @riginal Filed OCL. 21, 1948 4 Sheets-Sheet 4 ALTERNA 7'/N6 ALTERA///V CURRE/V C CURRENT- .SL/QCE I .Sol/,eca i TUTGT W Q; IN EN TOR. BY

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United States Patent Oflce 3,07,0Z6 Patented 06f. 3l, 1961 3,tll7,tl26 ELECTRICAL HEATING DEVICES George V. Woodling, 96? Union Commerce Eidg., Cleveland, (Ehio Original application Get. 21, 1948, Ser. No. 55,791, now

Patent No. 2,673,917, dated Mar. 3l), 11954. Divided and this application Sept. 21, 1953, Ser.. No. 331,310

2 Claims. (Cl. 219-25) The invention relates in general to electrical heating appliances and more particularly to household heating appliances which may be controlled by a controllable spaced discharge device yand which utilizes a metallic iilm impedance for the heating element.

An object of the invention is to provide a household electric liatiron with rectified alternating current energy from a rectifier controlled by a temperature responsive impedance in heat exchange relationship with the flatiron.

Another object of the invention is to provide a temperature responsive impedance in heat exchange relationship with a household electrical heating appliance to affect a bridge circuit and thus control a controllable rectifier supplying energy to the appliance, and further to place such impedance in circuit relation with the energy output of the rectifier.

Another object of the invention is to provide a household electric heating appliance wherein the heating element is a metallic resistor film deposited on a dielectric coating which is in turn supported by a metal carrier.

A still further object of the invention is to provide a household electric appliance adapted to be heated by a metallic film heating element with such film deposited on a dielectric coating and wherein the metallic Ifilm is supplied with electrical energy from a controllable rectiier, and a control circuit has a temperature responsive limpedance in heat exchange relationship with the heating element to control the controllable rectifier.

A still further object of the invention is to provide a household electric appliance adapted to be heated by a heating element supplied with electrical energy from a controllable rectifier, and a control circuit has a temperature responsive element in heat exchange relationship with the heating element to control the controllable rectier, and wherein either or both of the elements may be a metallic film.

Other objects land a fuller understanding of this invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a plan View of a metallic carrier supporting a dilectric coating which in turn supports a metallic film impedance used `as a heating element;

yFIGURE 2 is a cross-sectional view of FIGURE 1 showing the metallic film in exaggerated thickness;

FIGURE 3 is a sectional elevational view of a fiatiron incorporating a metallic film heating element;

FIGURE 4 is a sectional plan view taken on the line 4-4 of FIGURE 3;

FIGURE 5 is a partial sectional elevational view of a modified form of flatiron incorporating a metallic film impedance;

FIGURE 6 is a circuit diagram of a controllable supply circuit for the heating element;

`FIGURE 7 is a vector diagram of the voltages obtainable from the circuit of FIGURE 6;

1FIGURE `8 is another controllable supply circuit for the heating element;

FIGURE 9 is a further modification of a controllable supply circuit;

FIGURES l0 and 1l are still further modifications of a controllable supply circuit;

FIGURE 12 is a circuit diagram of a controllable energization circuit which incorporates a portion of the heating element in the phase shift bridge control circuit;

FIGURE 13 is a vector diagram of the vectors obtainable from circuit of FIGURE 12;

FIGURE 14 is a modification of the circuit of FIG- URE 12;

FIGURE 15 is a vector diagram of the vectors obtainable from the circuit of FIGURE 14;

FIGURE 16 is a controllable energization circuit for a heating appliance having a control impedance in the heating appli-ance; and

FIGURE 17 is a vector diagram of the vectors obtainable from the circuit of FIGURE 16.

This application is a division of my copending applioation, Serial No. 55,791, filed October 2l, 1948 which `matured in-to Patent Number 2,673,917, issued March 30, 1954.

FIGURES 1 and 2 illustrate one form of the invention wherein a metallic carrier 21 has recessed portions 22 and the entire upper surface 23 is covered with a dielectric or vitreous enamel coating 24. Terminal plates 25 are adapted to be attached or imbedded in the dielectric coating 24 at the recessed portions 22 and remain iixedly in place. A metallic film impedance 26 is adapted to be deposited in any well-known manner upon the dielectric coating 24 and terminal plates 25. A second dielectric coating 27 is adapted to cover the metallic film impedance 26 and to extend down over the sides to the metallic carrier 21 to act as electrical insulation and mechanical protection to the metallic film impedance 26. The manner of applying the metallic film impedance and the manner of applying and type of dielectric coating does not form part of the invention. The upper surface 23 ofthe second dielectric coating 27 is preferably made of a smooth surface so that the entire heating unit may be more satisfactorily utilized; for instance, the heating unit might be used as a hot plate or other appliance wherein a smooth working surface would be desirable. Electrical connection to the metallic film impedance is made by the terminal plates 25 since these terminal plates are directly in contact with the metallic film impedance 26. Terminal wires 77 may be attached to the terminal plates by any Well-known means such as soldering or brazing, and in the FIGURES 1 and 2 have been shown as being attached to the underside of the terminal plates 25 and gaining access to such terminal plates through holes 7S in the metallic carrier 21.

The FIGURE 2 shows the layers of dielectric coating and metallic ilm in cross section, and it is to be understood that this cross-sectional View is considerably exaggerated, especially las to the thickness of the metallic film impedance.

The FIGURES 3 and 4 show a metallic lm impedance as applied to a household heating appliance shown as an electric flatiron 29. The flatiron 29 includes a metal soleplate 30 having la working surface 31 and an opposite surface 32. A dielectric coating such as a vitreous enamel coating 33 is adapted to cover the opposite surface 32 of the metal soleplate 30 to act as an insulator. Such dielectric or vitreous enamel coating 33 may be applied in liquid state and may be such coating that is fired to a hard, glossy surface covering the opposite surface 32. A metallic film impedance 34 may be deposited on the dielectric coating 33 as the heating element of the ilatiron 29. A second dielectric coating 35 covers the metallic film impedance 3'4 to provide physical protection to the metallic film impedance 34 and to act as an electrical and heat insulator, The heating element may be made in substantially the same manner as is used in the trade in making thin metallic film resistors. The vitreous enamel physically protects the thin metal resistor film from abraacer/,oas

sion and moisture. With this type of construction, the entire electrical appliance may be washed without damage to the heating element and thus is rendered sanitary. The resistor lm may have zero, negative or positive temperature coefficient. Metal terminal plates 36 and 37 have been shown at the toe and heel portions of the flatiron 29 to provide electrical connection to the metallic film impedance 31S. These metal terminal plates 36 and 37 have been shown as being placed on the rst dielectric coating 33 whereupon the metallic film impedance 3d is deposited on these metal terminal plates as well as on the first dielectric coating 3?). The metal terminal plates 35 and 37 provide sufiicient thickness of metal to provide electrical connection to the wires 35 and such as by soldering or brazing at the terminals tl. rhe wires 3S and 39 may be joined into a cable 41 having a conventional male electrical plug 42.

A solid heat insulating cover 43 may cover the entire opposite surface 32 of the soleplate 39. This cover i3 may be of any solid insulator such as a plastic or any other well-known insulating substance. The cover has a handle portion X44. The cover 43 is adapted to be fastened to the soleplate 3f? by cap screws i5 which threadedly engage projecting lugs 4o on the opposite surface 32 of the soleplate 35 and preferably the cap screws 45 are recessed in the cover 43 as at 47.

The fiatiron shown in FIGURES 3 and 4 is an improved form of atiron since the use of the metallic fum impedance 34- permits lightweight construction of the flatiron, and also permits a minimum thickness between the working surface 3l and the upper surface i3 of the cover 43.

The FIGURE shows a portion of a flatiron Mft that differs in some particulars from the fiatiron 29 of FiG- URES 3 and 4. A plate 14E may be made of metal as is the usual custom and has a recess 142 to receive a terminal plate M3. Between the terminal plate 143 and the recess 142 is a dielectric coating 144 which completely covers the lower surface of the plate fit1. A metallic film impedance i452' is deposited on the dielectric coating lll-4l and is, hence, in electrical contact with the terminal plate M3. A second dielectric coating lido is deposited on the metallic film impedance 145' and also preferably covers the toe portion M7 of the plate Mil as well as the entire side edges of the plate Mi, for electrical insulation and for mechanical protection to the metallic film impedance ltd-5. By providing a flat surface to the first dielectric coating 144 the metallic film impedance M5 and the second dielectric coating 146 will also have a fiat surface to thus provide a smooth working surface for the flatiron 149. Such fiat surface may be obtained by grinding or other suitable method to eliminate any unevenness. A hole MS is provided in the plate lill so that a terminal Wire M9 may gain access to the terminal plate 143 and be electrically connected thereto in any suitable manner.

The circuit of l1EIGURE, 6 shows an energization circuit 53 which may be controlled in electrical output for energizing the heating element of a household electrical appliance. A household heating appliance Slt has been shown in dotted lines to indicate a flatiron having a heating element 5?). The energization circuit 53 is preferably housed in a separate housing as indicated by the dashed line 54. The energization circuit 53 includes generally a transformer 55' having a primary S6 energizable from an alternating current source 57 through the switch 58. A secondary 59 of the transformer 5S energizes the anodes 6@ of space discharge devices 6l. These space discharge devices have been shown as gaseous discharge tubes having a control element such as control grids 62. The space discharge devices have been shown as constituting a full wave rectifier system having a rectified alternating current output deliverable across the output terminals 53 and 64. The rectified output of the rectifiers 61 is delivered to the heating element 52 for energization thereof.

The energization circuit 53 also includes a control circuit 65 having a phase shift bridge 66 energized from a transformer winding 67. The phase shift bridge 66 has four arms with the `first arm including a manually variable resistance and a capacitive element 69. The second arm includes an impedance 75 shown as a resistance. The third arm is a temperature responsive imi edance 7 that is shown in heat exchange relationship with the heating element 52. In this case, the temperature responsive impedance 7l is shown as being enclosed within tie confines of the Flatiron 511.. The fourth arm of the bridge includes another impedance shown as resistance The output of the bridge 66 at terminals nd 'ie is supplied to a grid transformer 73 to variably the phase of the grid-cathode voltage relative to The vector diagram of FIGURE 7 may be referred to as an aid in understanding the operation of the circuit of .FIGURE 6. in all the following vector diagrams the voltage vectors will be given a reference character corresponding to the reference cl.arac'ter of the voltage source or impedance across which the voltage drop occurs. Similarly, a point potential will be given a reference character corresponding to the reference character of the terminal, juncture or pct in the circuit. The vector o7 designates the alternating current input voltage to the bridge which will be in phase with the alternating current voltage applied to the anodcs 60. The vectors 71 and 72 lie along the vector 67. The juncture 74 between the impedances 7i and 7?: is shown as the point 74 on the vector diagram of yFfGURlE 7. The juncture 7e between the capacitance element 69 and impedance 7@ is shown on the vector diagram by the reference character 7o. The first arm 75 of the bridge o@ which includes the variable resistance 63 and the capacitance clement is shown on the vector diagram of FIGURE 7 by the vector 7S. Similarly, the impedance 75 has a vector 7@ on the vector diagram. The output voltage of the phase shift bridge oo that is applied to the grid transformer 73 is shown by the vector output. The direction of the vector shown on this vector diagram indicates that the output voltage lags the input voltage o7 by an angle approximately 90 degrees. rhis would permit the rectiiiers 6l. to trigger or fire at a time phase 90 degrees lagthe anode-cathode voltage. The variable resistance may be manually adjusted to shift the location of the point 76 to thus adjust the firing angle ofthe space discharge device 6l. and hence adjust the rectified output to the heating element 52. The temperature responsive irnpedance 7l which is in heat exchange relationship with the heating element S2 should have a positive temperature coefiicient such that as the heating element 52 tends to overheat, the impedance of the temperature responsive impedance 7l. will increase to decrease the firing angle of the rectifier 6l and hence decrease the electrical output to the heating element 52. Thus, the phase shift bridge 66 of the control circuit 65 maintains a substantially constant temperature of the heating element 52.

FEGURE 8 she-ws a modification of the energization circuit of FIGURE 6. in this case, the rectifier circuit has been shown as a half wave rectifier circuit 79 that supplies energy to the heating element S2 of a heating appliance 5l which has again been shown as a atiron. A transformer winding 67 again supplies energy to a phase shift bridge 8u. This phase shift bridge 30 is shown as having first and second temperature responsive impedances 8f and S2 positioned in heat exchange relationship with the heating element 52. The juncture 83 between the impcdances Si and 32 is connected to one line 34 of the rectifier circuit 79. The connection between this juncture 3 and the line 84 has been shown as being made within the flatiron 51 in order that only four wires need be connected to the flatiron 5l. The phase shift bridge Sil has first and second arms 85 and S6 with a juncture 87 therebetween. The output of the bridge 80 is between the junctures 83 and S7 and applied to the cathode and grid of the rectifier of the rectifier circuit 79. The first temperature responsive impedance S1 preferably has a positive temperature coeicient and the second temperature responsive impedance 82, a negative temperature coefficient. By so providing positive and negative temperature coefficients the bridge 30 will be approximately twice as sensitive as the bridge 60 of the circuit of FIGURE 6. It will be obvious that the first temperature responsive impedance may have any given temperature coefficient and the circuit Will operate properly if the second temperature responsive impedance 82 has a temperature coefficient that is more negative or less positive than said given temperature coefiicient.

The vector diagram for the circuit of FIGURE 8 will be essentially the same as the vector diagram of FIGURE 7 except that there will be two temperature responsive impedances that vary with temperature changes rather than only one. Y

The circuit of FIGURE 9 shows a still further modification of a controllable energization circuit 90 having a half wave rectifier circuit 91. In this case, the heating element 52 is again supplied with energy from the rectifier circuit 91, but the heating element 52 is not in the household appliance which has been shown as a cordless fiatiron 92. The heating element is mounted within a container 93. The container 93 may be considered as a hot plate for heating the appliance or flatiron 92 and maintaining same at a substantially constant temperature as long as this fiatiron 92 is in contact with the hot plate surface 94 of the container 93. The arrangement shown in the circuit of FIGURE 9 may well be used for cordless automatic fiatirons or as a heating surface for any type of appliance such as a hot plate, oven or grill of a stove. The controllable energization circuit 90 includes a phase shift bridge 95 having four arms 96, 97, 98 and 99, all of which have been shown as being in heat exchange relationship with the heating element 52 within the container 93. In order to make this phase shift bridge 95 as sensitive as possible, the impedances of the first and fourth arms 96 and 99 should have a positive temperature coefficient, and the impedances of the second and third Aarms 97 and 93 should have a negative temperature coefiicient. The vector diagram for the circuit of FIGURE 9 will be essentially the same as the vector diagram shown in FIGURE 7 except that all four impedances of the bridge are temperature responsive in order to make the bridge 95 approximately four times as sensitiveas the bridge 66 of the circuit of FIGURE 6. The arm 98 is shown as having a variable condenser 100 therein to permit manual adjustment of the operating temperature of the heating element 52. Obviously, a variable resistor may be utilized for this purpose as in circuits described above; however, the variable condenser may have advantages of not being affected by the heat produced in the container 93 since it will have no movable contact surfaces as is the usual case with variable resstors. Further, the variable condenser may have a temperature coefficient other than zero, and a negative temperature coeliicient would still further increase the sensitivity of the bridge 95.

The circuit of VFIGURE also shows a controllable energization circuit 102 having a rectifier 103 for supplying rectified alternating current energy to the heating element 52. A household appliance 104 has been shown as a cooking vessel adapted to be placed in heat exchange relationship with the heating element 52 as by placing this cooking vessel 104 on a heating surface 105 of a container 106 which contains the heating element 52. The cooking vessel 104 has been shown as having a control impedance 107 incorporated into this cooking vessel 104 which control impedance 107 is one arm of a phase shift bridge 108. The bridge 108 controls the firing angle 6 of the rectifier 103. The control impedance 107 has a temperature coefficient other than zero in order to control the output of the phase shift bridge 108 and, hence, control the output of the rectifier 103. In the circuit as shown, the control impedance 107 should have a negative temperature coefiicient. The control impedance may take many forms and preferably is a metallic film impedance such as shown in the FIGURES 1-5. Metallic film impedances have been developed which may have the temperature coeflicient thereof controlled to a very high degree, and thus a metallic film impedance having a very large positive or negative temperature coeflicient may be selected for use as the control impedance 107. Such a metallic film impedance may be incorporated into the household appliance 104 in a manner similar to that shown in FIGURES 1 and 2 or the method shown in FIGURES 3, 4, and 5. Electrical connections to such metallic film impedance may be easily effected and these connections and the metallic film impedance themselves could be made water-tight and to present a smooth surface so that the household appliance 104 may easily be washed and kept in a sanitary condition.

The circuit of FIGURE ll shows a still further energization circuit 111 wherein a controllable rectifier 112 supplies rectified alternating current energy to a heating element 52. The rectifier 112 is controlled by a control circuit 113 which includes a fixed phase shift supplied by the resistance 114 and the capacitance 115. The resistance 114 has been shown as being variable to Vary the output of the rectifier 112. A thermocouple 116 has been shown as being in heat exchange relationship with the heating element 52, and the output of this thermocouple 116 is supplied to an amplifier 117 which amplifies the voltage obtained from the thermocouple 116 and applies it to the rectifier 112. The controllable rectifier 112 is controlled by a system known as a D.C. bias- A.C. rider system wherein the resistance and capacitance 114 and 115 supply a fixed phase shift of approximately ninety degrees lagging the anode voltage and the amplied voltage from the thermocouple supplies a variable direct current for varying the tiring angle of the rectifier 112. The therrnocouple 116 has been shown as being mounted within the heating appliance in close proximity to the heating element 52 so that it is in heat exchange relationship with this heating element 52. It will be obvious that this thermocouple 116 may be mounted in a separate unit such as the arrangement shown lin FIG- URE 10.

The circuit of FIGURE l2 shows a still further con trollable energization circuit 119 for supplying rectified alternating current energy to a heating element 120. The heating element 120 has an intermediate terminal 121, and the right-hand portion 122 of the heating element 120 serves the dual function of a portion of the heating element and also as one arm of a phase shift bridge 123. 'Ihe controllable energization circuit 119 includes a rectifier device 124 for supplying the rectified current to the heating element 120 and this rectified alterna 'ng current is preferably filtered by a lter 125 so as to apply essentially pure direct current to the heating element 120. The phase shift bridge 123 has condensers 126 and 127 in two arms 128 and 130 of this bridge 123 to prevent the direct current from flowing in this bridge 123. The alternating current impedance of the right-hand portion 122 will then be that used as one arm of the bridge 123.

The vector diagram of FIGURE 13 shows the vectors obtainable from the circuit of FIGURE 12. The first arm 12S of the bridge 123 is shown by the vector 128, and likewise the second and third arms 129 and 130 are shown by the vectors 129 and 130. The input voltage to the phase shift bridge 123 is shown by the vector input, and the output voltage of the bridge 123 is shown by the vector output. The right-hand portion 122 of the heating element 120 should be responsive to temperature changes acer/,cee

and, in the circuit as shown, should have a positive temperature coefficient to make a stable circuit. The second arm 129 has been shown as being variable in order to adjust the output of the rectifier 124 and hence the temperature of the heating element 1211.

The heating element 120 may be constructed of a metallic film impedance with an intermediate terminal conneet-ed thereto in order to supply an electrical connection as at the intermediate terminal 121.

The circuit of FIGURE 14 shows an improvement over the circuit of FIGURE 12 wherein only two leads are required to the heating element 131. The heating element 131 is again supplied with rectified alternating current energy from a rectifier 132 as filtered by a filter 133. The heating element 131 has been shown as being inductive which will not impede the flow of the direct current from the rectifier 132; however, it will impede the fiow of alternating current applied to it from a phase shift bridge 154. This phase shift bridge 134 includes first, second, and third arms 135, 136, and 137, and the heating element 131 constitutes the fourth arm of this bridge 134.

The vector diagram of FIGURE 15 shows the vectors obtainable from the bridge 134 wherein the arm 131 may have a high alternating current impedance relative to the first and second arms 135 and 136, and hence this bridge 134 may be made quite sensitive. An `advantage of the circuit of FIGURE 14 is that the heating element 131 performs the dual function of heating and control of the rectifier 132. No intermediate terminals are needed and hence merely the two end terminals of the heating element 131 need exist. The heating element 131 would thus be desirable for a portable heating appliance such as a flatiron which may then have merely the two normal wires for electrical connection to such heating element and need not have any extra wires or any separate control impedance.

FIGURE 16 is a further modification of the invention wherein a heating element 152 is supplied with rectified alternating current energy from the rectifier 153. The heating element 152 is adapted to be mounted in a base unit 154 which may have ears 155 as an aid in centering the fiatiron 156 on the base unit 154. The flatiron 156 has a control impedance 157 contained therein and adapted to be in heat exchange relationship with the heating element 152 when the fiatiron 156 is placed on a heating surface 158 on the base unit 154. The rectifier 1'53 has a bridge circuit 159 for control of the rectifier 153 and the control impedance 157 is adapted to be placed in parallel with one arm 161B of the bridge circuit 159. The electrical connection between the control impedance i157 and the bridge circuit 159 is provided by terminal plates 161 in the atiron 156 and terminal plates 162, in the base unit 154. The terminal plates 161 are electrical insulated from the flatiron 156 by insulators 163 and the terminal plates 162 are insulated from the base unit 154 by insulators 164. The terminal plates 161 and 162, may make electrical connection by surface contact or preferably by a plug and jack connection 165 and 166.

When the flatiron is placed upon the heating surface 158 so that the terminal plates 161 and 162 are in electrical contact, then the control impedance 157 is in parallel with the arm 160 to reduce the total impedance in this arm of the bridge circuit 159. This is arranged to cause the phase of the grid-cathode voltage to lag to a greater degree the phase of the anode-cathode voltage of the rectifier 153. This will reduce the electrical output of the rectifier 153 and reduce the heat produced by the heating element 152. A fourth arm 169 of the bridge 159 is made adjustable to provide for the manual variation of the operation temperature of the fiatiron 156. This will be the case when the fiatiron 156 is in heat exchange relationship with the heating element 152 and the control impedance 157 is part of the bridge circuit 159. When the atiron 156 is removed from the base unit 154,

the control impedance 157 will no longer be in parallel with the arm 161i and, hence, the impedance of such arm will increase to increase the electrical output of the rectifier 153. The heating element 152 will then raise in temperature in accordance with the increased electrical input thereto. When the fiatiron 156 is again placed on the base unit 152, the control impedance 157 will be in shunt with the arm 161i to reduce the output of the rectifier 153 to a value which will produce the desired temperature setting as dictated by the variable arm 169. However, the stored heat in the base unit 152 will rapidly raise the temperature of the flatiron 156 to this operating tempera ture. The control impedance 157 should preferably be of a negative temperature coefcient so that it will have a regulating effect upon the operation of the bridge circuit 159 and rectifier 153. Second and third arms 167 and 1625 of the bridge 159 may have a positive temperature coefcient to control the bridge 159 when the fiatiron 156 is removed from the base unit 154 and hence control the temperature of the base unit 152, and in such case the arms 167 and 163 should be in heat exchange relationship with the base unit 152.

The FGURE 17 shows the vector diagram for the circuit of FIGURE 16 wherein 160 is the voltage vector for the arm 160 and output is the output voltage of the bridge circuit 159. When the control impedance 157 is not in shunt with the arm 16u, the impedance of the arm 16d will increase to that shown by the dotted line vector 1611 and the output voltage will shift to a position less lagging with respect to the input voltage.

Any of the temperature responsive elements 71, 31, 82, 96, 97, 9S, 99, v116, 122, 131, 157, 167, and 168, just as the sensing element 107 may be in the form of a metallic impedance film, and constructed similarly to that shown in FIGURES 1 to 5, either as an individual film or in combination with a separate film as the heating element.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

l. A household electric fiatiron comprising a metal soleplate having a work surface and an opposite surface and having head and toe portions, a layer of insulation substantially covering one of said surfaces, a metallic film disposed adjacent said layer of insulation and having an area substantially the same as said soleplate, insulation means comprising vitreous enamel bonded to said metal plate and covering said metallic film and excluding said metallic film from external exposure, and means for providing electrical connection to said metallic film at said heel and toe portions.

2. An electric iron including a first layer of dielectric material having first and second sides, said second side comprising a work surface, a metallic film having a first and second side and substantially covering said first layer of dielectric material with said first side of said film adjacent said first side of said first layer of dielectric material, said film having first and second spaced portions, a first and a second electrical conductor for conducting electric current, a first conductor material connecting the first conductor to the metallic film at the first spaced portion of said metallic film and a second conductor material connecting the second conductor to the metallic film at the second spaced portion of said metallic film, a second layer of dielectric material having a first and a second side and substantially covering said metallic film with said first side of said second layer of dielectric material adjacent said second side of said metallic film, a support member having :a first side and substantially covering said second layer of dielectric material with said first side of said support member adjacent said second side of said second layer of dielectric material, passageway means providing space for exit of said irst and second conductors from said metallic film.

References Cited in the tile of this patent UNITED STATES PATENTS 522,718 Leonard July 10, 1894 729,369 Lowenthal May 26, 1903 910,725 Reimers Jan. 26, 19019 1,344,741 Thornton June 29, 1920 1,527,164 Anderson et al Feb. 24, 1925 1,563,731 Ducas Dec. 1, 1925 1,870,619 Flanzer Aug. 9, 1932 1,978,089 Jones Oct. 23, 1934 2,021,661 Kisfaludy Nov. 19, 1935 2,205,543 Rideau et al. June 25, 1940 10 Jira Sept. 5, 1944 Stong July 5, 1949 Burton July 4, 1950 Russell May 15, 1951 Del Buttero July 15, 1952 Sparklin et al. Apr. 21, 1953 Lytle Aug. 11, 1953 Dickey Dec. 29, 1953 Woodling Mar. 30, 1954 FOREIGN PATENTS Great Britain Aug. 23, 1948 Germany July 16, 1941 Australia May 20, 1937 OTHER REFERENCES Standard Handbook for Electrical Engineers, McGraw- Hill Book Co., New York City, N.Y., 1915, page 372. 

